The present invention relates to the field of polishing, in particular to the field of chemical mechanical polishing. More particularly, the invention relates to methods and apparatus for controlling and optimizing chemical mechanical polishing processes and materials for polishing substrates used in the manufacture of semiconductor wafers and integrated circuits.
Polishing processes play significant role in modern technologies, in particular in semiconductor fabrication. For example, at certain stages in the fabrication of devices on a substrate, it may become necessary to polish the surface of the substrate before further processing may be performed. In a polishing process, known as mechanical polishing, a polishing pad with abrasive particles repetitively passes over the surface of the substrate. Polishing may also be performed with a chemically active abrasive slurry. A polishing system that uses a chemical slurry is commonly known as a chemical mechanical polishing (CMP) system. In contrast with mechanical polishing, the slurry in a CMP system provides an increased removal rate of a substrate material. Additionally, by selecting particular chemicals, chemical slurry can be used to selectively polish certain films on a semiconductor substrate.
Chemical mechanical planarization, commonly referred to as CMP, may be used as a preparation step in the fabrication of substrates or semiconductor wafers to provide substantially planar front and back sides thereon. CMP is also used to remove high elevation features, or other discontinuities, which are created on the outermost surface of a wafer during the fabrication of microelectronic circuitry on the substrate.
The planarization method typically requires that the substrate be mounted in a wafer head or carrier, with the surface of the substrate to be polished exposed. The substrate supported by the head is then placed against a moving polishing pad mounted on a platen. The head holding the substrate may also rotate, to provide additional motion between the substrate and the polishing pad surface. Further, a polishing slurry (typically including an abrasive and at least one chemically reactive agent therein, which are selected to enhance the polishing of the topmost film layer of the substrate) is supplied to the pad to provide an abrasive chemical solution at the interface between the pad and the substrate. Pressure may be applied between the carrier and the platen to effectuate polishing. In some CMP machines the wafer rotates while the polishing pad is stationary, in others the pad moves while the wafer carrier is stationary, and in yet another type both the wafer carrier and the pad move simultaneously. The polishing pad may be pre-soaked and continually re-wet with a slurry that may have a variety of abrasive particles suspended in a solution of chemicals.
U.S. Pat. No. 5,597,341 issued on Jan. 28,1997 to Kodera, et al, U.S. Pat. No. 5,234,867 issued on Aug. 10, 1993 to Schultz, et al., and U.S. Pat. No. 5,232,875 issued on Aug. 3, 1993 to Tuttle, et al illustrate several techniques and corresponding types of CMP systems for chemical mechanical planarization of the semiconductor wafer surfaces.
One type of CMP systems is shown schematically in FIG. 1a. In this system a polishing pad 10a is mounted on a platen 12a, which rotates by means of a first motor 14a through a transmission 16a. A wafer 20a with a front surface 22a to be polished is held on a head 24a. In the illustrated apparatus, the polishing pad 10a has a diameter significantly larger than that of the wafer 20a (FIG. 1a). The polishing head 24a is rotated by means of a second motor 26a through a transmission 28a and comprises a retaining ring 30a, which prevents the wafer from slipping out of the head during polishing. A slurry feeding system 32a pours a slurry on the top working surface of the pad 10a. 
FIG. 1b illustrates another embodiment of the aforementioned known CMP system. In this embodiment, a polishing pad 10b is mounted on a platen 12b, which is rotated by means of a first motor 14b through a transmission 16b. A wafer 20b with a front surface 22b to be polished is held on a head 24b. In the illustrated apparatus, the polishing pad 10b has a diameter significantly smaller than that of the wafer 20b (FIG. 1b). The polishing head 24b is rotated by means of a second motor 26b through a transmission 28b and comprises a retaining ring 30b, which prevents the wafer from slipping out of the head during polishing. A slurry feeding system 32b pours a slurry on the front surface of the wafer 22b. 
In order to provide uniformity of polishing, in the CMP systems of the types shown in FIGS. 1a and 1b, the distance between the polishing pad rotational axis and the wafer rotational axis is typically varied in an oscillatory manner. For this purpose, the substrate is repeatedly moved back and forth relative to the polishing pad. In FIGS. 1a and 1b the oscillatory movement is shown by arrows 25a and 25b, respectively.
Another type of the CMP system, shown schematically in FIG. 2, is disclosed, e.g., in U.S. Pat. No. 5,899,800, issued on May 4,1999 to Shendon and in U.S. Pat. No. 6,184,139, issued on Feb. 6, 2001 to Adams et al. In the CMP apparatuses of these patents, the lower head comprising a polishing pad 10c mounted on a platen 12c is driven into orbital movements by means of an orbital drive 34 with a motor 36, while the carrier 24c holding the wafer 20c rotates about the central axis O1xe2x80x94O1 by a motor 26c via a transmission 28c. The pad diameter is slightly larger than the diameter of the wafer 20c. A polishing fluid (slurry) is introduced to the wafer directly through the openings 38a, 38b, . . . 38n in the polishing pad 10c with point-of-use mix, which results in better wafer uniformity and reduced slurry consumption.
The efficiency of polishing greatly depends on the pad surface conditions and may reduce with time as polishing pad is worn out. Therefore in the course of polishing, the pad surface should be refreshed or xe2x80x9cconditionedxe2x80x9d after a period of use to provide for a more uniform polishing rate, from wafer to wafer, and to provide for better planarization uniformity across a single wafer. During the pad conditioning process, a pad conditioner arm with an abrasive lower surface is forced to come in contact with the pad upper surface while the pad rotates or oscillates and the conditioner arm moves back and forth on the polishing pad. While the operation of conditioning is an effective way of deterring the wear of the polishing pad, the pad requires replacement if its surface conditions are not recovered by conditioning.
Important characteristics of a planarization process in semiconductor wafer fabrication are a removal rate, uniformity, and end point detection (EPD). Removal rate is the rate of material removal from the surface of semiconductor wafer being polished. Preferably, the rate of removal should be such that any surface peaks are preferentially flattened and the resultant surface is as near perfectly planar as possible. Uniformity of the material removal over the entire wafer surface is critical in order to provide required flatness and to avoid over- or under-polished areas on the wafer. Detection of the end point (i.e. the moment when predefined degree of surface flatness or non-uniformity is attained and polishing process has to be terminated) is usually associated with polishing of wafers with multiple layers on the surface, when the uppermost layer has to be completely or partially removed to expose the next layer. It requires very accurate determination of transition from one layer to another.
There are several factors that may affect all the above-mentioned parameters.
Since various materials of the upper layer on a wafer, polishing pad, slurry, and retaining ring interact in a course of polishing, the combination of polishing pad and retaining ring characteristics, the specific slurry mixture, and other polishing parameters, such as compression force or contact pressure between wafer and polishing pad, rotational and/or linear speed, temperature, etc., can provide specific polishing characteristics. Thus, for any material being polished, the pad and slurry combination is theoretically capable of providing a specified finish and flatness on the polished surface. For example, the nature of the slurry can have a dramatic effect. The slurry includes abrasive particles suspended in a solvent, which selectively may soften certain features of the pattern on the semiconductor wafer surface, thereby affecting the relative rate of removal of those features. It must be understood that additional polishing parameters, including the relative speed between the substrate and the pad and the force pressing the substrate against the pad, affect the polishing rate, finish, and flatness.
Therefore, for a given material whose desired finish is known, an optimal pad, retaining ring, and slurry combination may be selected. Typically, the actual polishing pad and slurry combination selected for a given material is based on a trade off between the polishing rate, which determines in large part the throughput of wafers through the apparatus, and the need to provide a particular desired finish and flatness on the surface of the substrate.
By nature, the removal of material during polishing is caused by interaction or friction between wafer, pad, and slurry particles. Polishing process, in particular a CMP process, to a great extent depends on such factors as friction characteristics of the material being treated, surface conditions of the treated material and the polishing pad, friction forces in the zone of contact of the polishing pad with the treated material, characteristics of the polishing slurry, the rate of wear of the polishing tool, the rate of removal of the material from the treated surface, etc.
Theoretical determination of friction forces and torques on parts participating in a polishing process, in particular in CMP, is extremely difficult, if possible at all for following reasons:
1) The friction force in the zone of contact between the relatively moving parts and a torque that occurs on these parts are functions of the aforementioned parameters of the polishing process.
2) The friction force in the zone of contact between parts participating in a relative motion has a non-linear dependence on the relative velocity between the parts. FIG. 3 shows a typical dependence of the friction force FFR between wafer and pad in the presence of fluid versus relative speed V.
3) The parts in contact participate simultaneously in three motions such as two rotations and one relative linear motion.
4) In the course of polishing, the polishing pad and the workpiece being treated constantly change their properties, e.g., due to variation in thickness and surface properties.
For the above reasons, for control and optimization of industrial technological processes, it is necessary to experimentally determine the friction properties of various materials used in real polishing conditions, as well as a friction behavior of interacting parts during the CMP process.
Since CMP systems of all types provide substantial flexibility in selecting materials used and parameters (variables) to control the polishing process, the manner in which materials and parameters are selected and optimized can be overwhelming.
Determination of the variables and selection of the materials for polishing a particular substrate in a particular manner typically is accomplished by estimation coupled with trial and error testing on a number of substrates. Such trial and error testing can consume an inordinate amount of time and materials before an appropriate combination of variables is found. Presently, there are no apparatus or methods available for controlled polishing and simulating a CMP system to determine the variables necessary to provide a particular polishing characteristic. Additionally, there are no methods or apparatus for optimizing variable values to achieve a particular polishing characteristic within a minimal amount of polishing time.
Another known apparatus for simulating and optimizing a CMP system is disclosed in U.S. Pat. No. 5,599,423, issued on Feb. 4, 1997 to Parker et al. The simulated CMP system disclosed therein comprises a polishing pad, a chuck for supporting a substrate, a positioner for positioning the polishing pad relative to the substrate (or vice versa), a chuck rotator for rotating the chuck, and a polishing pad rotator for rotating the polishing pad. The CMP system simulator is implemented as a computer program that is executed on a general purpose computer system. The simulator enables a user to enter particular simulation parameters that define polishing pad size and shape, substrate size, polishing pad dwell time at particular locations on the substrate, pad aging, pad to substrate pressure, rotational velocity of the pad relative to the substrate, and a number of passes of the substrate over the pad. From these parameters, the simulator calculates polishing results that indicate the amount of substrate material removed during polishing of the substrate.
In fact, though the apparatus disclosed is capable to simulate the operation of a CMP system and to calculate the results of a polishing process based on a set of predefined motion parameters and average data of material properties, it doesn""t take into account changes of real material properties (such as removal rate, friction coefficient) and their variations during polishing, as well as pad conditioning. Also it doesn""t allow for measuring forces, torques, and deformations in interacting parts in the course of a polishing process.
There is also known a method of polishing and planarizing semiconductor devices as disclosed in U.S. Pat. No. 5,036,015 xe2x80x9cMethod of Endpoint Detection during Chemical/Mechanical Planarization of Semiconductor Wafersxe2x80x9d issued on Jul. 30, 1991 to Sandhu , et al. According to this method, the turntable of a CMP apparatus is driven to rotate by an electric motor, and changes in the friction between the wafer held by a wafer holding device on the turntable and the polishing pad for polishing the wafer are detected as changes in the electric current flowing through the electric motor.
Other examples of a CMP process control method based on measuring the running motor current in order to detect variations of the motor torque related to variations of mechanical parameters (such as friction force) in the zone of contact of the rotating pad with the surface being treated, are described, e.g., in U.S. Pat. No. 5,948,700 issued on Sep. 7, 1999 to Zheng, et al.
As described below, this known technique also is not applicable for accurate measuring forces ant torques and for polishing process control and optimization.
FIG. 4 illustrates a typical dependence of the electric current running through the electric motor versus a load or torque applied to the motor shaft. Since no load current Io flows through the electric motor when no load is applied thereto, it is difficult to accurately detect the level of friction developed on the platen. Furthermore, the current flowing through the motor greatly depends on the voltage of corresponding power supply and on speed of rotation. Therefore even small variations in the power supply voltage and changes in the rotation speed cause significant changes in the current.
In addition, since in the aforementioned CMP system both the platen and the wafer holding device are connected to respective motors through corresponding transmissions, accuracy of friction measurements based on the motor current may be affected by losses and slippage in the transmissions.
A method and apparatus for controlling a polishing process described in U.S. Pat. No. 5,738,562 issued on Apr. 14, 1998 to Doan, et al. are based on measurement of variations that occur in translational (lateral) motions of the polishing platen, related to the variations in friction coefficients of different film materials. These method and apparatus are based on indirect measurement technique, result in very approximate evaluation of the friction variations, cannot accurately measure the friction coefficients and thus, are not suitable for practical control of the CMP process.
There are also another known polishing apparatus and method for planarizing a layer on a semiconductor wafer, as disclosed in U.S. Pat. No. 5,948,205, issued on Sep. 7, 1999 to Kodera et al. According to the disclosure, the above method comprises steps of measuring friction between the layer being polished and a turntable carrying a polishing slurry during polishing, determining the polishing rate from the measured friction, determining the extent of polishing by integrating the polishing rate over time, and terminating the polishing operation when the measured polishing extent coincides with a predetermined value. More specifically, the polishing apparatus disclosed in this patent, comprises means for measuring friction developed between the layer being polished and a turntable carrying a polishing slurry during the polishing operation, determining the rate of polishing the layer based on the measured friction and determining the extent of polishing of the layer by integrating the polishing rate over time.
The above method and apparatus are based on the assumption that xe2x80x9cthe friction between the layer being polished and the turntable carrying a polishing slurry and the rate of polishing the layer show a relationship of one-to-one correspondence.xe2x80x9d
By utilizing this relationship, the authors of the aforementioned patent propose to measure the friction caused between the layer being polished and a turntable carrying a polishing slurry during the polishing operation, determine the rate of polishing the layer from the measured friction, determine the extent of polishing of the layer by integrating the polishing rate with time, and terminate the polishing operation upon coincidence of the extent of polishing of the layer with a predetermined value.
The disclosed apparatus further comprises a system of measuring the distortion of the shaft connected to the polishing turntable to determine the load due to friction caused at the turntable and converting the measured value into an electric signal to control the operation of the electric motor for driving the turntable.
A tester and a method for measuring individually various friction characteristics, such as friction forces, torques, and normal compression forces between relatively moving parts are known and described in pending U.S. patent application Ser. No.09/624,500 filed on Jul. 24, 2000 by the same applicants. This universal friction tester for testing tribological properties of materials comprises a frame with a carriage sliding in vertical guides and supporting a slide moveable in a horizontal direction. The slide supports a stationary upper specimen, which engages a moveable lower specimen, located in a replaceable module attachable to a base plate of the frame. The modules may be of a rotary, reciprocating, a block-on-ring, or any other type, required for different test conditions. Testing can also be carried out with heating or with the supply of oil or other fluid in the zone of contact between the specimens.
Although the tester described above is suitable for testing and measuring various tribological characteristics of materials, these tester and the method are not applicable for simulating real CMP conditions, since only one of two contacting parts rotates, and a slurry having certain viscosity can behave differently from real polishing conditions. Furthermore, the known method and tester provide measurement of a friction torque only on one of the parts, i.e., on the stationary part, which is unsuitable for CMP where both parts participate in rotation. Another disadvantage of the known method and apparatus is that the test is conducted without pad conditioning, which does not simulate real CMP conditions. As described above, the operation of conditioning helps to refresh the polishing pad surface in order to keep polishing rate constant and uniform. With known polishing methods, the timing of conditioning or replacing the pad in most cases is determined on the basis of experience of the operator or other empiric basis. This means that the rate of polishing a semiconductor wafer is not accurately controlled and therefore cannot be kept at a constant level.
An apparatus and a method for conditioning and monitoring media used for chemical-mechanical planarization are known, as disclosed in WO Pat. No. 01/15865 A1, issued on Mar. 8, 2001 to Moore. According to the disclosure, a CMP machine contains a conditioning body attached to a support and a force sensor connected to the conditioning body support for measuring a friction force developed in the interface between the conditioning body and a polishing pad. The apparatus allows monitoring and controlling of a conditioning process. However, as has been stated above and is shown in the aforementioned patent, the frictional force can be a function of the surface characteristics of the pad and/or of the conditioning tool, as well as a function of the normal compression force and the relative velocity between the two surfaces. Therefore, a demand for more accurate control of the pad conditioning still exists.
Thus, the known methods and apparatus do not provide full control and measurement of real friction characteristics inherent in a CMP process. Therefore, the prior-art technique is not suitable for complete control and optimization of the CMP process and for selection of most optimal pairs of materials for friction under specific operation conditions.
It is an object of the present invention to provide effective, accurate, universal, and reliable method and apparatus for a controlled polishing process such as CMP. Another object is to provide a method and apparatus, which control CMP processes on the basis of combined direct mechanical, acoustical, and thermal measurements of polishing conditions. It is another object to provide a method and apparatus for directly measuring a friction coefficient in a CMP process under various operation conditions and with the use of different polishing materials. Yet another object is to provide an apparatus and method for a CMP process with controlled conditioning of the polishing pad surface.
The invention provides an apparatus for a controlled polishing process which is capable of simultaneously measuring compression and friction forces, developed in contact between an object to be polished (e.g., a semiconductor wafer) and a polishing pad, and torques developed on the object and pad. The apparatus comprises a rigid frame with a base and a vertical column, a rotational polishing head, which can be positioned vertically and horizontally, and holds an object, and a polishing pad with a rotary or orbital drive installed on the frame under the polishing head. The polishing head is attached to the positioning mechanism via force sensors and a torque sensor, while another torque sensor can be placed between the polishing pad and the frame to which it is attached. Another sensor, measuring a compression force between the object and the pad and corresponding friction response (force or torque), can be installed between the head and an object holder (e.g., a wafer carrier). In the process of polishing, the polishing head rotates together with the object in contact with the polishing pad and at the same time performs radial movements with respect to the center of the polishing pad, while sensors simultaneously measure corresponding forces and torques. A data processing unit of the apparatus receives the data signals from the sensors and computes process parameters, such as friction coefficient, removal rate, etc. The apparatus makes it possible to find operation conditions most optimum for specific speeds of the head and pad with reference to materials of the wafer, pad and polishing slurry.
Additionally, the invention provides the aforementioned apparatus and a method, wherein groups of high-frequency acoustic emission sensors are built into components of the rotating head and of the object holder, so that in addition to measuring compression and friction, the apparatus also measures high-frequency acoustic emission signals corresponding to changes that occur in the interface between the object and the pad. All groups of sensors work simultaneously and their measurement data are processed and analyzed by a data acquisition, processing and control units for obtaining accurate and reliable results. The analysis of these signals allows to control a polishing process more effectively and accurately and to obtain better polishing results.
In addition, the apparatus of the invention is equipped with a pad conditioner (of a brush or abrasive type) is attached to the force sensors measuring compression and friction between the conditioner and the pad. The data processing unit receives the data signals from these sensors, computes parameters of conditioning, and controls polishing process for obtaining repeatable and accurate results.